An imaging device, such as a digital camera, converts an image signal output from an imaging element, such as CCD type, CMOS type, to a brightness signal (Y) and a color difference signal (Cb, Cr), and records the converted image signal into an external recording medium, such as a memory card.
The image signal contains a lot of noise when the amount of light incident to one pixel is reduced due to high pixelation of an imaging element or when sensitized imaging is executed by night, for example. One method of reducing the noise is to utilize various spatial filters using the value of a noted pixel and the values of peripheral pixels around the noted pixel (for example, see JP-A-2-249365). A low pass filter for removing high frequency components, a median filter adopting a median value, etc. are known as the spatial filters. These spatial filters are particularly effective for noise from high frequency components of brightness components.
Another method of reducing the noise is to determine the color saturation of an image from the color difference signal (Cb, Cr) and make the color difference signal (Cb, Cr) variable with respect to an image having low brightness and color saturation on the basis of the brightness signal (Y) and the color saturation (for example, see JP-A-2006-332732). This method is more effective to color shift noise in which colors different from the original color are speckled, as compared with the spatial filter. This is because it is estimated that noise based on the color shift contains more low frequency components than noise based on the brightness component, and thus it is distributed in a broad range.
However, in connection with the high pixelation of the imaging element requirement and the highly sensitized imaging requirement, observation of the noise based on the color shift is not limited to the dark portion, but expands over the whole area of the brightness level. However, according to this method, the color difference signal is made variable with respect to an image having high brightness and saturation for adaptation, and thus the value of the color difference signal is varied as a whole, so that a color indicated by a signal to be stored is displaced from the color of a subject, that is, color shift occurs over the whole area of the color difference level. Accordingly, in the case of the above method, it is difficult to reduce the color shift noise.
Furthermore, the image signal having the large amount of noise described above also has a large amount of noise in the brightness signal. Therefore, the above method may induce new color shift noise correlated to the brightness noise because its processing is dependent on the brightness.